The present invention relates to methods for processing sonic waveform measurements, particularly sonic waveform measurements made for the purpose of characterising properties of underground formations. The invention in particular relates to methods for determining the best value for a parameter that has been determined in a number of different ways.
It has been know for some time that it is possible to determine properties of underground formations using measurements of acoustic/sonic waves that have passed through the formations. The basic technique comprises placing a tool comprising a spaced sonic source and receiver in the borehole and using the source to generate sonic waves which pass through the formation around the borehole and are detected at the receiver. Sonic waves can travel through rock formations in essentially two forms: body waves and surface waves. There are two types of body waves that travel in rock: compressional and shear. Compressional waves, or P-waves, are waves of compression and expansion and are created when rock formation is sharply compressed. With compressional waves, small particle vibrations occur in the ,same direction the wave is travelling. Shear waves, or S-waves, are waves of shearing action as would occur when a body is struck from the side. In this case, rock particle motion is perpendicular to the direction of wave propagation. The surface waves are found in a borehole environment as complicated borehole guided waves which come from reflections of the source waves reverberating in the borehole. The most common form or borehole-guided, surface wave is the Stoneley wave. FIG. 1 shows a series of sonic waveforms such as would be recorded in a borehole from a monopole (omnidirectional) source with the first arrivals of the compressional (P), shear (S) and Stoneley (St) waves at the receiver marked. In situations where dipole (directional) sources and receivers are used, an additional shear/flexural wave propagates along the borehole and is caused by the flexing action of the borehole in response to the dipole signal from the source The flexural wave typically travels at about the same speed as the shear wave, slower than the compressional wave. (It is to be noted that sonic waves will also travel through the fluid in the borehole and along the tool itself. With no interaction with the formation, these waves carry no useful information and run on wireline or coiled tubing or the like, or alternatively can be a logging while drilling tool located in a drill string being used to drill the borehole.
The speeds at which these waves travel through the rock are controlled by rock mechanical properties such as density and elastic dynamic constants, and other formation properties such as amount and type of fluid present in the rock, the makeup of the rock grains and the degree of intergrain cementation. Thus by measuring the speed of sonic wave propagation in a borehole, it is possible to characterise the surrounding formations by parameters relating these properties. The speed or velocity of a sonic wave is often expressed in terms of 1/velocity and is called xe2x80x9cslownessxe2x80x9d. Since the tools used to make sonic measurements in boreholes are of fixed length, the difference in time (xcex94T) taken for a sonic wave to travel between two points on the tool is directly related to the speed/slowness of the wave in the formation.
An example of a tool for use in a borehole for sonic measurements is the DSI tool of Schlumberger which is shown schematically in FIG. 2. The DSI tool comprises a transmitter section 10 having a pair of (upper and lower) dipole sources 12 arranged orthogonally in the radial plane and a monopole source 14. A sonic isolation joint 16 connects the transmitter section 10 to a receiver section 18 which contains an array of eight spaced receiver stations, each containing two hydrophone pairs, one oriented in line with one of the dipole sources, the other with the orthogonal source. An electronics cartridge 20 is connected at the top of the receiver section 18 and allows communication between the tool and a control unit 22 located at the surface via an electric cable 24. With such a tool it is possible to make both monopole and dipole measurements. The DSI tool has several data acquisition operating modes, any of which may be combined to acquire (digitised) waveforms. The modes are: upper and lower dipole modes (UDP, LDP)xe2x80x94waveforms recorded from receiver pairs aligned with the respective dipole source used to generate the signal; crossed dipole modexe2x80x94waveforms recorded from each receiver pair for firings of the in-line and crossed dipole source; Stoneley modexe2x80x94monopole waveforms from low frequency firing of the monopole source; P and S mode (PandS)xe2x80x94monopole waveforms from high frequency firing of the monpole transmitter; and first motion modexe2x80x94monopole threshold crossing data from high frequency firing of the monopole source.
One way to determine compressional, shear and Stoneley slownesses from these measurements is to use slowness-time-coherence (STC) processing. STC processing is a full waveform analysis technique which aims to find all propagating waves in the composite waveform. The processing adopts a semblance algorithm to detect arrivals that are coherent across the array of receivers and estimates their slowness. The basic algorithm advances a fixed-length time window across the waveforms in small, overlapping steps through a range of potential arrival times. For each time position, the window position is moved out linearly in time, across the array of receiver waveforms, beginning with a moveout corresponding to the fastest wave expected and stepping to the slowest wave expected. For each moveout, a coherence function is computed to measure the similarity of the waves within the window. When the window time and the moveout correspond to the arrival time and slowness of a particular component, the waveforms within the window are almost identical, yielding a high value of coherence. In this way, the set of waveforms from the array is examined over a range of possible arrival times and slownesses for wave components. STC processing produces coherence (semblance) contour plots in the slowness/arrival time plane. Regions of large coherence correspond to particular arrivals in the waveforms. The slowness and arrival time at each coherence peak are compared with the propagation characteristics expected of the arrivals being sought and the ones that best agree with these characteristics are retained. Classifying the arrivals in this manner produces a continuous log of slowness versus depth. For dispersive waves, the STC processing is modified to take into account the effect of frequency. As the output of STC processing is a coherence plot, the coherence of each arrival can be used as a quality indicator, higher values implying greater measurement repeatability. When processing dipole waveforms, one of the coherence peak will correspond to the flexural mode but with a slowness that is always greater (slower) than the true shear slowness. A precomputed correction is used to remove this bias.
To compensate for variations in measurements due to the borehole rather than due to the formation a series of measurements are made across an interval in which the formation properties are expected to vary little, if at all. In its simplest form, the interval corresponds to the extent of the receiver array, and the waveforms at each receiver station measured for a given firing of a source (xe2x80x9creceiver arrayxe2x80x9d or xe2x80x9creceiver modexe2x80x9d or xe2x80x9cRec.xe2x80x9d). In simple STC processing, all receiver stations are considered. In multishot STC processing (MSTC), sub-arrays of receiver stations within the receiver array are considered, for example a sub-array of five receiver stations in a receiver array of eight receiver stations (other numbers or receiver stations in the sub-array can be used depending on requirements). In this case, the same formation interval corresponding to the extent of a five receiver station sub-array can be measured several times as the tool is logged through the borehole, the five stations making up the sub-array being selected at each source firing to measure the same formation interval. Another approach, known as xe2x80x9ctransmitter modexe2x80x9d or xe2x80x9cpseudo-transmitter arrayxe2x80x9d (xe2x80x9cTra.xe2x80x9d) takes waveforms from sequential source firings as the transmitter passes along the interval to be measured. In order to compensate for the movement of the tool between measurements, an effectively stationary receiver station or sub-array must be used. This can be achieved by changing the receiver station considered so that its position in the borehole is effectively stationary as the transmitter is moved through the interval. Borehole compensation (xe2x80x9cBHCxe2x80x9d) can be achieved for P and S mode results by processing receiver array and pseudo-transmitter array waveforms and averaging the results.
Thus it will be appreciated that, with the different acquisition modes of a tool such as the DSI, and the different processing modes available, it is possible to obtain multiple determinations of a slowness or xcex94T in a given interval of borehole. For example, it is possible to acquire waveforms for shear slowness determination using two dipole modes (upper dipole and lower dipole), and one monopole mode (P and S mode), and to process each measurement in receiver mode, transmitter mode and borehole compensated form resulting in a potential nine separate determinations of shear slowness for that interval, each of which can give a different result. The problem is therefore to determine which slowness estimate can be considered to give the best indication of the shear slowness of the formation in that interval.
The present invention provides a method of determining the sonic slowness of an underground formation from sonic measurements, comprising: (i) obtaining sonic waveforms in the underground formation; (ii) determining, from the sonic waveforms, multiple values of at least one parameter related to the sonic slowness of the formation together with an estimate of the error in each value; and (iii) using the estimate of the error in each value to select a parameter value related to slowness as representative of the formation.
The sonic waveforms are preferably obtained by logging an interval of a borehole which runs through the formation with a tool which outputs sonic waveform measurements. The tool can be run on wireline or coiled tubing or the like, or alternatively can be a logging while drilling tool located in a drill string being used to drill the borehole.
The multiple values of the parameter related to sonic slowness can include multiple determinations of monopole and/or dipole compressional and shear (including flexural) slowness, and Stoneley slowness for the formation. Where the tool used to obtain the waveforms comprise an array tool, the multiple determinations can include transmitter and receiver mode measurements and borehole compensated measurments. The processing technique used is preferably a slowness-time-coherence technique.
The processing technique can include amongst its inputs, zoning information derived from the waveform measurements and indicating general features of the formation type being measured. The zoning information can be obtained from a basic compressional slowness estimation, typically based on digital first arrival determination, and can include broad formation slowness classifications such as fast, slow, very slow and extremely slow. Such broad classifications can be based on predetermined cross plots of the ratio of compressional and shear slownesses against measured slowness for know lithologies. Other zoning information can be the presence of closely spaced bed boundaries (thin beds).
Other inputs to the processing technique can include parameters relating to the borehole or well, such as hole diameter, mud type and predetermined formation features.
The specific processing technique applied can vary according to the zoning or parameter inputs.
For example, where zoning information shows relatively thick beds, full array STC-type processing (including dispersive processing) can be applied; if zoning information shows thin beds, high resolution, sub-array multishot STC-type processing can be applied. The processing also preferably includes tracking of slowness measurements along the interval so as to allow individual slowness measurements to be associated with changes in particular components of the waveform (P-waves, S-waves, etc.). The tracking can also make use of the zoning information indicating where major changes occur, and delineating homogenous beds, each with associated semblance error bars.
The output of the processing step is a series of slowness estimates and the tracking can also include the estimation of error in the value of slowness. An estimate of error can be provided by the statistical semblance processing of the slowness measurements. The total error is a combination of the semblance error and the uncertainty in tracking determination. The final step selects the slowness with the minimum error (possibly modified by other predetermined selection rules) and outputs this as the slowness for the interval. For example, the mean error for the interval can be used as the basis for selection. Also, a level by level determination of the variation of the actual error from the mean within the interval can be provided as a further output